Antihistamine drugs are a group of medications that prevent the level of histamine from rising in our body and reduce the symptoms this can cause. Antihistamines can be used to treat a range of conditions including allergic reactions, motion sickness and insomnia. However, the term "antihistamines" most commonly refers to medications that are used to treat hay fever. Antihistamines that are available to buy over-the-counter in the UK include: Piriton® (the active drug is chlorphenamine), Clarityn (the active drug is loratadine), Piriteze (where the active drug is cetirizine) and Benadryl® allergy relief (the active drug is acrivastine). These are generally licensed for the treatment of hay fever, allergic reactions to pets, dust mites and food, insect bites, chickenpox rash and prickly heat rash. They are available as creams, tablets and oral liquid.
If you are suffering from hay fever or allergic-type symptoms such as an itchy nose, itchy rash or bite on the skin or redness and minor swelling of the skin, then an antihistamine may help relieve these symptoms for you. Minor allergic reactions to products (washing detergent, creams, perfume), food or plant reactions (stinging nettles) can be safely treated with over-the-counter antihistamines and should get better in hours to days. Make sure to avoid whatever caused the reaction in future. If you find that you are getting regular allergic reactions and do not know the reason why, you should discuss with your doctor, and you may benefit from getting allergy testing.
Histamine is a naturally occurring chemical in your body that is activated and released from your cells when your immune system is triggered by a foreign substance. Histamine sets off a chain reaction, allowing your immune system to detect and remove the thing that caused it. Whilst a useful and important response, the release of histamine causes redness, swelling and itchiness that can be bothersome. Individuals that suffer from hay fever or show allergic sensitivity to harmless stimuli have a higher than normal amount of histamine released by the immune system. Antihistamines help to prevent and reduce the effect of histamine in the body, and therefore are effective at relieving allergic symptoms. Antihistamines can be classified as drowsy or non-drowsy. Chlorphenamine (Piriton) can make you drowsy. Care should be exercised when driving or operating heavy machinery, and alcohol should be avoided. Some antihistamines that cause drowsiness are also found in over-the-counter sleeping tablets such as diphenhydramine. All other over-the-counter antihistamines are classed as non-drowsy: cetirizine, loratadine and acrivastine. These are less likely to cause drowsiness, but it is still a possible side effect.
Different medications have different doses and schedules for how often you can take them. Check with your pharmacist or read the box carefully if you are not sure. Most antihistamines are safe to take for long periods of time, for example throughout hay fever season, however chlorphenamine should not be used for longer than two weeks at a time without consulting a doctor. Antihistamine creams are also available to buy and may be more suitable for treating localised reactions such as insect bites. Talk to your doctor if you are pregnant or breastfeeding before taking antihistamines. Do not take any antihistamines three days before taking an allergy skin test as this can affect your results. Like all medications, don’t take them if you have previously had an allergic reaction to the medication, if you have severe kidney or liver problems, or if you have other medications or medical conditions where you would normally discuss with your doctor or pharmacist before starting something new.
Common side effects include drowsiness. These are more common with chlorphenamine, but may also be experienced in non-drowsy antihistamines too. Other side effects include dizziness and headache.